1. Field of the Invention
This invention relates to ignitor disablers for vapour discharge lamps, and more particularly, to ignitor disablers which are suitable for disabling ignitors employed igniting and operating circuits for High Intensity Discharge (HID) lamps, particularly of the high pressure sodium type.
2. Background of the Prior Art
High intensity discharge (HID) lamps are used in many applications because of their long life and high efficiency for converting electrical energy to light. The principal types of HID lamps are mercury vapour, metal halide and high pressure sodium (HPS).
Mercury vapour, metal halide and HPS lamps all operate similarly during stabilized lamp operation. The visible light output results from the ionization of gases confined within an envelope and which must be broken down before there is any flow of ionization current. For this reason, a high open circuit voltage must be applied to a HID lamp for igniting and this voltage is substantially higher than the operating voltage and the available line voltage.
Another characteristic of HID lamps is that they exhibit negative resistance; that is, when operating, their resistance decreases with increase in the applied voltage. As a result, such devices require an impedance means in their power supply circuit for limiting the current flow to a predetermined value.
Because of the high starting or igniting voltage requirement and the negative resistance characteristic, HID lamps are provided with igniting and operating circuits which provide a relatively high open circuit voltage and impedance means for current limitations. A ballast between the power supply and the lamp typically services as the impedance means in igniting and operating circuits for HID lamps. For some HID lamps, such as mercury vapor lamps, igniting voltages are on the order of two times the operating voltage. The igniting voltage is generated by the ballast acting in conjunction with a capacitor. For other types of HID lamps, such as HPS lamps, wherein the required igniting voltages are typically more than ten times the operating voltages, more complex igniting mechanisms are required. Examples of starting or igniting circuits for such lamps are disclosed in U.S. Pat. No. 4,322,660 to Johnson and U.S. Pat. No. 4,683,404, to Hitchcock. The Johnson patent discloses an apparatus for providing high voltage pulses for starting a HPS discharge lamp, the apparatus having a single capacitor in series with a blocking diode and a charging resistor. When the voltage of the capacitor reaches a predetermined voltage exceeding the zener voltage of a parallel zener diode, the capacitor discharges through a ballast which is connected in autotransformer relationship therewith to provide the high voltage pulse to start the lamp. Hitchcock discloses an apparatus including two capacitors, two blocking diodes, a voltage sensitive symmetrical switch, and multiple resistances across which pulses are distributed. The aforementioned elements are electrically connected together and with a tapped ballast reactor so that one of the capacitors charges through an impedance in the negative half-cycle, and thereafter, when line voltage goes positive the other capacitor charges through an impedance equal to the sum of the multiple resistances. When the voltage of the capacitors reaches a predetermined voltage exceeding the breakdown voltage of the voltage sensitive symmetrical switch, the capacitors discharge. This discharge, because of an autotransformer relationship within the reactor, produces a high voltage pulse of predetermined height and width once per each cycle of the source voltage.
Notwithstanding the many similarities noted above between the various types of HID lamps, there are, however, unique requirements for HPS lamps which the ballast system typically provides. As discussed in the preceding paragraph, some type of electronic ignitor is used in conjunction with the HPS ballast coils to produce a high voltage pulse to start the HPS lamp. In all cases, these electronic ignitors work on the principle of sensing whether or not the lamp is burning, and if not, the ignitor continuously supplies starting pulses to the lamp. Electronic ignitors are generally insensitive to reasons why the lamp is not burning, and, accordingly, function in the same manner regardless of whether the non-burning of a lamp is caused by lamp failure, by absence of a lamp in the lamp socket, or by the lamp "cycling" off. Lamp cycling is a well known phenomenon in which a lamp nearing the end of its life will light, burn for some time, go out, relight and repeat this cycle time after time, until the lamp is either replaced or will fail to start at all. This phenomenon is due to the characteristic life trend of the lamp operating voltage in a HPS lamp. As a HPS lamp nears the end of its life, its lamp operating voltage gets so high that the ballast will no longer sustain operation. This condition usually manifests itself as an above-described "cycling" lamp. Further information about this phenomenon is set forth in an article entitled "Recommendations for Lamp Maintenance in High Pressure Sodium Luminaries" in Main-Lighter--Official Publication of the INTERNATIONAL ASSOCIATION OF LIGHTING MAINTENANCE CONTRACTORS, Volume 10, Number 7, page 1 (December 1982).
From the foregoing, it should be clear that there are a number of shortcomings in the prior art. In certain cases, e.g. when a lamp is cycling, failed or missing, the ignitor in the lamp's HID circuit continues to operate. Such operation shortens ignitor life, particularly in cases where the ignitor operates in conjunction with the ballast so that more than normal excitation power is drawn by the ballast transformer; that is, to be more specific, where the ballast coil is stressed as a consequence of the ignitor operation. Cycling lamps can also cause problems by possibly avoiding easy detection of impending failure. A lamp may be "cycling on" when inspected and, hence, escape replacement. In situations where obtaining access to lamps is difficult and inspections are not frequent, such as when the lamps are used for roadway lighting, failure to detect a cycling lamp will inevitably lead to futile ignitor operation and consequent deterioration.